Dyeing Mechanism

Affinity

It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.

Substantivity

The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.

Reproducibility of Shades

The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.
Characteristics of highly reproducible dyes are:

• Highly soluble


• Medium substantivity


• Medium reactivity


• Good wash off properties


• Highly diffusible

Optimization of Dye

The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.

1.Substrate

• Affinity


• Circulation speed


• Action of chemicals before

2.Dyestuff

• Depth of shade


• Optimum quantity/yield


• Diffusion ability and regularity


• Color fastness


• Combination & mixability


• Chromphore percentage

3.Auxiliary Products

• Optimum quantity


• Compatibility with dyestuff and with each other


• Levelness


• Control of PH in final exhaustion


• Reproducibility


• No adverse effect

4. Temperature and time

• Low initial temperature to avoid rapid absorption of dye


• Control of critical temperature zone for maximum exhaustion


• Sufficient time for penetration and fixing

5. Machine

• Control of batch


• Volume of flow


• Temperature regulation

The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.

Functional Groups of fiber

Cotton: OH-, at higher pH it is ionizable
 

Wool: -COOH, -NH, -CONH2. At pH 3-4 ionized positively so acid dye is used to dyeing
 

Acrylic: -COOH, -SO3H, -O SO3H
 

Silk: -NH2, -CONH
 

Viscose: -OH, -COOH
 

Polyester: -OH, -COOH. No ionization effect, high temperature used for dyeing with dispersing.
 

Diacetate: -OH, -COOCH3
 

Triacetate: -COOCH3

Dyeing Medium

1. Aqueous medium

• Water


• Solvent


• Foam

2. Vapor phase: cationic, anionic, nonionic

Dyeing Mechanism

The sequence of dyeing falls into four stages
1. Transfer of dye onto fiber surface
2. Adsorption
3. Diffusion into the fiber
4. Interaction

Transfer of dye onto fiber surface

The transfer of dye onto the fiber surface depends on:

1. Environment of the dyebath: environment of the bath refers to

• Solvent and its type, nature, quantity: solvent may be water and or any other solvents which may be soft/hard, acidic, alkaline, ionic, nonionic etc.


• pH


• Dyeing assistants like electrolytes, leveling agents, carrier, dispersing agents etc.


• temperature of the dyebath which depends on material type (cotton or polyester), type of dye (hot brand or cold brand), method of dyeing (padding or exhaust) Suitable environment ensures easy transference of dye on fiber surface.

2. Substantivity

3. Mechanical and physical force

Adsorption

The distribution process is called adsorption, if the substance which is to be distributed is retained by a surface. The assembly of dye molecules at the fiber surface is governed by:
Electropotential forces: All fiber when immersed into water or aqueous solution acquires an electric potential known as ‘zeta potential.’ Cellulosic fiber bears a negative charge while protein fibers at higher pH than its isoelectric point bears are negatively charged and at lower pH than isoelectric point is positively charged.
Temperature: most dyes in solution are either in molecular and partially ionized state or exist in the form of ionic micelles; increase in temperature tends to breakdown micelles into less aggregated units. Increase of temperature promotes vibrational activity accelerates the migration of the surface of the fiber.

Agitation: when a fiber is immersed in the dye a large no of molecules tend to enter the fabric at once, thus creating a layer called ‘Barrier.’ If the dye molecules are to reach the fiber surface then the barrier should be broken which is done by agitation.
Dye adsorption has affect on fastness properties.